Carbon doping semiconductor devices

ABSTRACT

A method of fabricating a semiconductor device can include forming a III-N semiconductor layer in a reactor and injecting a hydrocarbon precursor into the reactor, thereby carbon doping the III-N semiconductor layer and causing the III-N semiconductor layer to be insulating or semi-insulating. A semiconductor device can include a substrate and a carbon doped insulating or semi-insulating III-N semiconductor layer on the substrate. The carbon doping density in the III-N semiconductor layer is greater than 5×10 18  cm −3  and the dislocation density in the III-N semiconductor layer is less than 2×10 9  cm −2 .

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.61/791,395, filed on Mar. 15, 2013. The disclosure of the priorapplication is considered part of and is incorporated by reference inthe disclosure of this application.

TECHNICAL FIELD

This disclosure relates generally to fabricating semiconductor devices,and in particular to carbon doping semiconductor devices.

BACKGROUND

Many transistors used in power electronic applications have beenfabricated with silicon (Si) semiconductor materials. Common transistordevices for power applications include Si CoolMOS, Si Power MOSFETs, andSi Insulated Gate Bipolar Transistors (IGBTs). While Si power devicesare inexpensive, they suffer from a number of disadvantages, includingrelatively low switching speeds and high levels of electrical noise.More recently, silicon carbide (SiC) power devices have been considereddue to their superior properties. III-Nitride or III-N semiconductordevices, such as gallium nitride (GaN) based devices, are now emergingas attractive candidates to carry large currents, support high voltages,and to provide very low on-resistance and fast switching times.

SUMMARY

In one aspect, a method of fabricating a semiconductor device caninclude forming a III-N material structure in a reactor and, whileforming the III-N semiconductor layer, injecting a hydrocarbon precursorinto the reactor, thereby carbon doping the III-N semiconductor layerand causing the III-N semiconductor layer to be insulating orsemi-insulating.

In a second aspect, a semiconductor device can include a substrate and acarbon doped insulating or semi-insulating III-N semiconductor layer onthe substrate. The carbon doping density in the III-N semiconductorlayer is greater than 1×10¹⁸, 5×10¹⁸, or 1×10¹⁹ cm⁻³, and a dislocationdensity in the III-N semiconductor layer is less than 2×10⁹ cm⁻².

In a third aspect, a method of forming a semiconductor materialstructure can include forming a first III-N semiconductor layer on asubstrate in a reactor, and while forming the first III-N semiconductorlayer, injecting a hydrocarbon precursor into the reactor, therebycarbon doping the first III-N semiconductor layer and causing the firstIII-N semiconductor layer to be insulating or semi-insulating. Themethod can further include forming a second III-N material layer on thefirst III-N semiconductor layer, wherein the second III-N material layerhas a substantially lower carbon concentration than the first III-Nmaterial layer.

In a fourth aspect, a material structure can include a first III-Nsemiconductor layer on a foreign substrate, and a second III-Nsemiconductor layer on a side of the first III-N material structureopposite the foreign substrate, the second III-N semiconductor layerbeing thinner than the first III-N semiconductor layer. The first III-Nsemiconductor layer can have a carbon concentration greater than 1×10¹⁸cm⁻³ throughout the layer, and a carbon concentration throughout thesecond III-N semiconductor layer can be less than the carbonconcentration throughout the first III-N semiconductor layer.Furthermore, a surface of the second III-N material layer that isopposite the substrate can have a density of macroscopic features whichis less than 5 features/cm², wherein each of the macroscopic featureshas an average height of greater than 100 nanometers.

In a fifth aspect, a material structure can include a first III-Nsemiconductor layer on a foreign substrate, and a second III-Nsemiconductor layer on a side of the first III-N material structureopposite the foreign substrate, the second III-N semiconductor layerbeing thinner than the first III-N semiconductor layer. The first III-Nsemiconductor layer can be an insulating or semi-insulating layer havinga carbon concentration greater than 1×10¹⁸ cm⁻³. A carbon concentrationof the second III-N semiconductor layer can be less than the carbonconcentration of the first III-N semiconductor layer, and a dislocationdensity at a surface of the second III-N semiconductor layer oppositethe foreign substrate can be less than 2×10⁹ cm⁻².

Methods and devices described herein can each include one or more of thefollowing features. Injecting the hydrocarbon precursor can compriseinjecting a hydrocarbon precursor having a chemical formula(C_(x)H_(y)), where x and y are integers greater than or equal to 1.Forming the III-N semiconductor layer on the substrate can compriseforming the III-N semiconductor layer as a III-N buffer layer over aIII-N nucleation layer over a silicon substrate. Methods can compriseforming a III-N channel layer over the III-N buffer layer and forming aIII-N barrier layer over the III-N channel layer, thereby forming atwo-dimensional electron gas (2DEG) active channel adjacent to aninterface between the channel layer and the barrier layer. Forming theIII-N semiconductor layer as a III-N buffer layer can comprise formingthe III-N buffer layer under a plurality of growth conditions, andforming the III-N channel layer can comprise forming the III-N channellayer under the same or substantially the same growth conditions. Theplurality of growth conditions can comprise a surface temperature and areactor pressure. The plurality of growth conditions can furthercomprise a ratio of group-III precursor flow rate to group-V precursorflow rate. Forming the III-N semiconductor layer on the substrate cancomprise forming the III-N semiconductor layer by metal organic chemicalvapor deposition (MOCVD). The barrier layer can comprise AlGaN, thechannel layer can comprise undoped or unintentionally doped (UID) GaN,and the buffer layer can comprise AlGaN or GaN or both.

Forming the III-N semiconductor layer can comprise injecting a group-IIIprecursor into the reactor at a group-III precursor molar flow rate, andinjecting the hydrocarbon precursor into the reactor can compriseinjecting the hydrocarbon precursor into the reactor at a hydrocarbonprecursor molar flow rate, wherein the hydrocarbon precursor molar flowrate is at least 0.02 times the group-III precursor molar flow rate.Forming the III-N semiconductor layer can comprise injecting a group-IIIprecursor into the reactor at a group-III precursor molar flow rate, andinjecting the hydrocarbon precursor into the reactor can compriseinjecting the hydrocarbon precursor into the reactor at a hydrocarbonprecursor molar flow rate, wherein the hydrocarbon precursor molar flowrate is greater than the group-III precursor molar flow rate. Thehydrocarbon precursor can comprise propane or methane or both. Methodscan further comprise adding a gate terminal, a drain terminal, and asource terminal to the semiconductor device, thereby forming a III-Nhigh electron mobility transistor (HEMT). Methods can further compriseadding an anode terminal and a cathode terminal to the semiconductordevice, thereby forming a III-N diode. Causing the III-N semiconductorlayer to be insulating or semi-insulating can comprise causing the III-Nsemiconductor layer to have a resistivity of at least 1×10⁵ or 1×10⁷ohm-cm. Carbon doping the III-N semiconductor layer can result in theIII-N semiconductor layer having a carbon concentration greater than1×10¹⁸ cm⁻³. The hydrocarbon precursor can be injected into the reactorwhile forming the first III-N material layer but not while forming thesecond III-N material layer.

The III-N semiconductor layer can have a first side distal from thesubstrate and a second side between the first side and the substrate,wherein the dislocation density in the III-N semiconductor layer is adislocation density adjacent to the first side of the III-Nsemiconductor layer. The III-N semiconductor layer can comprise a III-Nbuffer layer over a III-N nucleation layer, wherein the substrate is asilicon substrate. Devices can further comprise a III-N channel layerover the III-N buffer layer and a III-N barrier layer over the III-Nchannel layer, thereby forming a two-dimensional electron gas (2DEG)active channel adjacent to an interface between the channel layer andthe barrier layer. The barrier layer can comprise AlGaN, the channellayer can comprise undoped or unintentionally doped (UID) GaN, and thebuffer layer can comprise AlGaN or GaN or both. The substrate can be aforeign substrate. Devices can further comprise a gate terminal, a drainterminal, and a source terminal, wherein the semiconductor device is aIII-N high electron mobility transistor (HEMT). Devices can furthercomprise an anode terminal and a cathode terminal, wherein thesemiconductor device is a III-N diode. The carbon doping density in theIII-N semiconductor layer can be less than 5×10²¹ cm⁻³.

A surface of the second III-N material layer that is opposite thesubstrate can have a density of macroscopic features which is less than5 features/cm², wherein each of the macroscopic features has an averageheight of greater than 100 nanometers. A combined thickness of the firstIII-N semiconductor layer and the second III-N semiconductor layer canbe less than 6 microns, for example less than 5 microns, less than 4microns, or less than 3 microns. The second III-N material layer can bethinner than the first III-N material layer.

Particular embodiments of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. An insulating or semi-insulating carbon dopedIII-N layer can be formed with a level of carbon doping from a widerange of concentrations (1e16-1e22 cm⁻³) with fewer restrictions on oneor more growth parameters of the layer compared to conventionaltechnology. Insulating or semi-insulating layers can be formed with lowdislocation densities and smooth surfaces grown on foreign substrates,e.g., Si or SiC substrates. Injecting a halogen free precursor (e.g., ahydrocarbon precursor) during metalorganic chemical vapor deposition(MOCVD) can reduce or eliminate interactions of halogen containingmolecules with the metalorganic precursors, thereby avoiding theinfluence of CX₄ (X=halogen) precursors on an alloy composition (i.e.,the ratio of Al to Ga in AlGaN) during MOCVD growth of carbon dopedAlGaN.

DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B are cross-sectional views of an example III-Nsemiconductor device.

FIG. 2 is a flow diagram of an example method for fabricating a III-Nsemiconductor device including a carbon doped layer.

FIG. 3 is a block diagram of a system for fabricating a III-Nsemiconductor device with at least one layer that is carbon doped.

FIG. 4 is a cross-sectional view of an example III-N semiconductormaterial structure.

FIGS. 5A and 5B are cross-sectional and plan view schematic diagrams,respectively, of a macroscopic feature formed on the surface of a III-Nmaterial structure.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

FIG. 1A is a cross-sectional view of an example III-Nitride (i.e.,III-N) semiconductor device 100. For example, the device can be atransistor, e.g., a III-N high electron mobility transistor (HEMT), byadding source 114, drain 116, and gate 118 terminals to the device, asillustrated in FIG. 1B. In another example, the device can be a diode byadding anode and cathode terminals to the device (not shown).

The device includes a substrate 102. The substrate can be, e.g.,silicon, SiC, aluminum nitride (AlN), GaN, sapphire (Al₂O₃), or anyother suitable growth substrate for the growth of III-N materials.Because large native substrates (i.e., substrates formed of III-Nmaterials) are currently unavailable and tend to be very expensive, thedevice is typically formed on a foreign substrate (i.e., a substrateformed of a material that is not a III-N material), such as silicon,silicon carbide, or sapphire. The device includes a nucleation layer 104on the substrate. The nucleation layer can be a III-N nucleation layerand can include, e.g., AlN.

The device includes a buffer layer 106. The buffer layer can be a III-Nbuffer layer and can include, e.g., C-doped AlGaN or GaN or both. Thebuffer layer can include several different layers, e.g., with somelayers closer to the substrate having a higher concentration of Al andsome layers further from the substrate having a lower concentration ofAl. The buffer layer can be made insulating or semi-insulating by carbondoping the buffer layer. This can be useful, e.g., to prevent subsurfaceleakage or premature breakdown.

The device includes a III-N channel layer 108 and a III-N barrier layer110, where the compositions of the channel layer and the barrier layerare selected to induce a two-dimensional electron gas (2DEG) 112 activechannel adjacent to an interface between the channel layer and thebarrier layer. For example, the channel layer can include undoped orunintentionally doped (UID) GaN and the barrier layer can include AlGaN.

The terms III-Nitride or III-N materials, layers, devices, andstructures can refer to a material, device, or structure comprised of acompound semiconductor material according to the stoichiometric formulaB_(w)Al_(x)In_(y)Ga_(z)N, where w+x+y+z is about 1, and w, x, y, and zare each greater than or equal to zero and less than or equal to 1. In aIII-Nitride or III-N device, the conductive channel can be partially orentirely contained within a III-N material layer.

The layers of the device can be formed by molecular beam epitaxy (MBE)or metalorganic chemical vapor deposition (MOCVD) in a reactor or othertechniques. One way to achieve carbon doping in a III-N layer formed byMOCVD with NH₃ as the nitrogen precursor is to adjust the layer growthconditions so that carbon from metalorganic precursors (e.g., TMGa orTMAl or both) is incorporated into the layers. For example, some growthconditions that favor the incorporation of carbon include: low reactorpressure, low NH₃ partial pressure, low deposition temperatures, andhigh growth rates.

When these growth conditions are implemented for carbon doping at levelssufficient to cause a layer to be insulating or semi-insulating forcertain applications, the growth conditions are limited for calibrationwith respect to other features of the layer, e.g., threading dislocationdensity and surface roughness of the layer. For example, consider alayer formed on a foreign (i.e., non-III-N) substrate, e.g., silicon(Si), silicon carbide (SiC), or sapphire (Al₂O₃).

Such a layer can be formed under growth conditions including one or moreof lower reactor pressure, lower NH₃ partial pressure, lower depositiontemperatures, and higher growth rates, but these growth conditions canalso result in higher dislocation densities and higher levels of pointdefects in the layer. Increasing carbon doping levels to greater thanabout 5×10¹⁸ cm⁻³ (and in some cases greater than 8×10¹⁷ cm⁻³) usingthese methods can additionally result in surface roughening or poorsurface morphology or both.

Another way to achieve carbon doping in a layer is to inject ahydrocarbon precursor into the reactor during growth of the layer.Hydrocarbon precursors include molecules of the chemical composition(C_(x)H_(y)), where x and y are integers greater than or equal to 1.Examples of hydrocarbons include propane (C₃H₈), methane (CH₄), andC₂H₂.

This way of achieving carbon doping can result in the layer havingcarbon doping in excess of 1×10¹⁸, 5×10¹⁸, 1×10¹⁹, or 3×10¹⁹ cm⁻³ whilesimultaneously having a dislocation density less than 2×10⁹ cm⁻², forexample about 1×10⁹ cm⁻² or less or about 8×10⁸ cm⁻² or less. The carbondoping density in the III-N semiconductor layer can be between 1×10¹⁹cm⁻³ and 5×10²¹ cm⁻³, or between 1×10¹⁸ cm⁻³ and 5×10²¹ cm⁻³. In someimplementations, the nucleation layer is between 20-50 nm thick, thebuffer layer is between 1-10 microns thick (e.g., about 5 microns), thechannel layer is about 200-1000 nm thick (typically about 400 nm), andthe barrier layer is between 10-40 nm thick (e.g., about 25 nm).

FIG. 2 is a flow diagram of an example method 200 for fabricating aIII-N semiconductor device including a carbon doped layer. For purposesof illustration, the method will be described with reference to theexample device 100 of FIG. 1, but the method can be used to fabricateother devices and to carbon dope other types of layers in other devices.

A nucleation layer is formed on a silicon substrate (202). For example,the silicon substrate can be placed into a reactor such as an MOCVDreactor, and the nucleation layer can be deposited, e.g., as a layer ofAlN within the reactor.

A buffer layer is formed on the nucleation layer (204). For example, thebuffer layer can be deposited, e.g., as a layer of AlGaN or GaN or both.In some implementations, the buffer layer comprises more than one layer.Layers of AlGaN are deposited, with a decreasing amount of Al in eachsuccessive layer. Eventually, one or more layers of GaN can bedeposited.

While the buffer layer is formed, a hydrocarbon precursor is injectedinto the reactor (206). For example, the hydrocarbon precursor can beinjected into the reactor simultaneously or alternately while injectinggroup III and/or group V precursors into the reactor.

A channel layer is formed on the buffer layer (208). For example, thechannel layer can be deposited, e.g., as a layer of undoped orunintentionally doped (UID) GaN. In some implementations, the channellayer is formed under the same or substantially the same growthconditions as the buffer layer. Where the buffer layer includes a toplevel layer of GaN, the channel layer can be deposited by ceasing toinject the hydrocarbon precursor and continuing to deposit GaN withoutaltering any other growth conditions in the reactor. That is, thereactor pressure and/or temperature and/or the total gas molar flow rateinto the reactor and/or the ratio of group V precursor molar flow rateto group III precursor molar flow rate can be the same for the channellayer and for the portion of the buffer layer that is directly adjacentto the channel layer, with a hydrocarbon precursor injected into thereactor during growth of the portion of the buffer layer that isdirectly adjacent to the channel layer but not during growth of thechannel layer.

A barrier layer is formed on the channel layer (210). For example, thebarrier layer can be deposited, e.g., as a layer of AlGaN. Atwo-dimensional electron gas (2DEG) active channel is induced adjacentto an interface between the channel layer and the barrier layer. Thebarrier layer can have a larger bandgap than the channel layer, whichcan in turn at least partially cause the 2DEG to be induced in thechannel layer. To form a transistor, source, gate, and drain terminalsare then formed on the III-N material layer structure (212).Alternatively, to form a diode, anode and cathode terminals are thenformed on the III-N material layer structure (not shown).

FIG. 3 is a block diagram of a system 300 for fabricating a III-Nsemiconductor device with at least one layer that is carbon doped. Thesystem can be used, for example, to perform the method of FIG. 2 tofabricate the device of FIGS. 1A and 1B.

The system includes a reactor 302, e.g., an MOCVD reactor. A substrate304 is placed into the reactor and a III-N layer 306 is formed on thesubstrate. A reactor control system 308 controls the formation of thelayer 306 by adjusting one or more growth conditions. The reactorcontrol system can control the injection of one or more materials intothe reactor, including carrier gases 316 (e.g., an inert carrier gassuch as H₂ or N₂ or both), group-V precursor gases 318 (e.g., NH₃),group-III precursor gases 320 (e.g., TMGa or TMAl or both), andhydrocarbon precursor gases 322 (e.g., one or more of C₃H₈, CH₄, andC₂H₂).

The reactor control system can be implemented, e.g., as a system of oneor more computers that receives input from an operator and providesoutput control signals, e.g., to the reactor and storage modules for thegases. The reactor control system can include a pressure control module310 (e.g., to control the pressure in the reactor), a depositiontemperature control module 312 (e.g., to control the surface temperatureof a layer being formed), a growth rate module 314, and other modules,for example. The growth rate module 314 may control the growth rateindirectly by controlling variables which affect the growth rate, suchas reactor pressure, surface temperature, and flow rates of the variousprecursors and carrier gases.

In some implementations, the reactor control system is configured toform the III-N semiconductor layer by injecting a group-III precursorinto the reactor at a group-III precursor molar flow rate and byinjecting the hydrocarbon precursor into the reactor at a hydrocarbonprecursor molar flow rate. The amount of carbon doping in the layer canbe at least partially controlled by varying the ratio between thehydrocarbon precursor molar rate and the group-III precursor molar flowrate.

It has been found that for some hydrocarbon precursors for carbon dopingof III-N materials during MOCVD growth of the III-N materials, inparticular propane (C₃H₈), the dopant incorporation efficiency is muchlower than the incorporation efficiency of other dopants typicallyintroduced during MOCVD growth of III-N materials. For example, for adopant such as silicon, where silane or disilane is used as the siliconprecursor, when the ratio of the silicon precursor molar flow rate tothe group-III precursor molar flow rate is about 1/1000 (and in somecases even lower), the silicon doping level in the III-N material isapproximately equal to the saturation limit of the dopant in the III-Nmaterial, which may be around 1×10²¹ cm⁻³. Increasing the siliconprecursor molar flow rate relative to the group-III precursor molar flowrate to a higher value does not substantially increase the concentrationof electrically active silicon in the layer, and typically results in apoorer structural quality of the resulting III-N layer, for exampleleading to higher dislocation and point defect densities, as well aspoor surface morphology. However, for carbon doping of III-N materialsduring MOCVD growth using propane as the carbon precursor, when thegrowth is performed under reactor conditions that correspond to lowcarbon doping levels (e.g., less than 1×10¹⁷ cm⁻³) in the absence of thepropane precursor, adding propane at a molar flow rate of about 1/1000that of the group-III precursor molar flow rate does not substantiallyincrease the carbon doping in the III-N material, and typically stillyields a carbon doping level which is less than 1×10¹⁷ cm⁻³.

In some systems, and in particular when propane (C₃H₈) is utilized asthe hydrocarbon precursor, a hydrocarbon precursor molar flow rate whichis about or at least 0.02 times the group-III precursor molar flow ratemay be needed in order for the carbon doping level in the layer to bebetween about 1×10¹⁷ and 1×10¹⁹ cm⁻³, or to be in excess of 1×10¹⁷ cm⁻³.In some systems, when the hydrocarbon precursor molar flow rate is aboutor at least 0.2 times the group-III precursor molar flow rate, thecarbon doping level in the layer can be about or in excess of 1×10¹⁸cm⁻³, or between about 1×10¹⁸ and 1×10²⁰ cm⁻³. In some systems, when thehydrocarbon precursor molar flow rate is substantially greater than thegroup-III precursor molar flow rate, e.g., 2 times or 20 times or 200times or 2000 times or 20,000 times the group-III precursor molar flowrate, the carbon doping level in the layer can be about or in excess of1×10¹⁸ or 1×10¹⁹ or 1×10²⁰ cm⁻³. The resistivity of a carbon doped layerformed with propane precursors can be greater than 1×10⁵ ohm-cm forcarbon doping levels of about 1e18 cm⁻³ or larger, or greater than 1×10⁷ohm-cm for carbon doping levels of about 1×10¹⁹ cm⁻³ or larger, orgreater than 1×10⁸ ohm-cm for carbon doping levels of about 1×10²⁰ cm⁻³or larger.

In some implementations, the reactor control system is configured toform at least one layer (e.g., the UID GaN channel layer) at a surfacetemperature of 1077 C and a pressure of 200 mBarr. The reactor controlsystem flows the nitrogen precursor, e.g., ammonia (NH₃), into thereactor at a rate of 0.54 mol/min, flows tri-methyl gallium (TMGa) intothe reactor at a rate of 0.65 milli-mol/min, and controls the total gasflow into the reactor to at or about 80 liters per minute. The reactorcontrol system can maintain the total gas flow at a substantiallyconstant rate by increasing or decreasing the carrier gas flow tocompensate for increases or decreases in other flows. This results incarbon doping of about 5×10¹⁶ cm⁻³ or lower in this layer.

The reactor control system can form the carbon doped layer under thesame or substantially the same growth conditions by flowing thehydrocarbon precursor into the reactor. For example, for the carbondoped layer, if the surface temperature is maintained at 1077 C, thepressure is maintained at 200 mBarr, the ammonia flow rate is maintainedat 0.54 mol/min, the TMGa flow rate is maintained at 0.65 milli-mol/min,and the rate of total gas flow into the reactor is maintained at about80 liters per minute, by flowing a hydrocarbon precursor into thereactor, carbon doping levels of greater than 1×10¹⁸ cm⁻³, greater than5×10¹⁸ cm⁻³, greater than 1×10¹⁹ cm⁻³, or greater than 1×10²⁰ cm⁻³ canbe achieved. At the same time, if the carbon doped III-N layer is formedon a foreign substrate such as silicon, the dislocation density of theupper portion of the carbon doped III-N layer (i.e., the portionadjacent to the surface of the carbon doped III-N layer which isfurthest from the substrate) can be maintained at a level smaller than2×10⁹ cm⁻², and typically even smaller than 1×10⁹ cm⁻², even if thetotal thickness of the III-N layers in the structure is less than 6microns, less than 5 microns, less than 4 microns, or less than 3microns.

By way of comparison, if the hydrocarbon precursor is not flowed intothe reactor during growth of the carbon doped layer, the reactor controlsystem can adjust one or more or all of the growth parameters toincorporate enough carbon to cause the carbon doped layer to becomeinsulating to a specified degree. For example, the reactor controlsystem can reduce the pressure to 50 mBarr, reduce the temperature to1020 C, reduce the NH₃ flow rate to 0.045 mol/min, maintain the totalgas flow at about 80 liters per minute, and maintain the flow ofgroup-III precursor gases.

These adjustments to the growth conditions can result in carbon dopingof up to about 5×10¹⁸ cm⁻³. The dislocation density at the upper surfaceof the layer when the layers are grown under these conditions can begreater than 2×10⁹ cm⁻², and is typically between 5×10⁹ and 6×10⁹ cm⁻².Further adjusting the reactor conditions to further increase the carbonconcentration in these layers can cause substantial degradation in thesurface morphology of the material structure, and typically also resultsin even higher dislocation densities.

Referring now to FIG. 4, in many III-N semiconductor devices, the activeportion of the device is contained within the layer 418 of the III-Nmaterial structure 420 which is furthest from the substrate 402. Forexample, referring to the transistor structure of FIGS. 1A and 1B, thedevice channel 112 is contained within the channel layer 108 (thus thechannel layer 108 and barrier layer 110 of FIGS. 1A and 1B correspond tothe additional layer 418 of FIG. 4). In such devices, it is oftenpreferable to electrically isolate the substrate 402 and/or nucleationlayer 404 and/or buffer layer 406 from the additional layer 418, whileforming the additional layer 418 under conditions that result in minimaldefects and/or traps in the additional layer 418. As previouslydescribed, this can be achieved by injecting a hydrocarbon precursorinto the reactor during growth of the nucleation and/or buffer layers404 and 406, respectively, in order to dope these layers with carbon andrender them insulating or semi-insulating, while growing some or all ofthe additional layer 418 as an undoped (or unintentionally doped) layer,with substantially lower levels of carbon. In many cases, the thicknessof the buffer layer 406 is greater than that of the additional layer418, such that at least half of the thickness of the III-N materialstructure has a substantial carbon doping. Such a structure can resultin a reduced dislocation density at the surface of the additional layer418, as well as causing the upper surface of the III-N materialstructure 420 to be substantially smoother, as compared to the casewhere the carbon doping of the nucleation and/or buffer layers isachieved by other methods. These improved characteristics result inimproved device performance and higher yields.

For example, when the carbon doping is achieved by other methods thatwere previously described, such as reducing the reactor pressure andtemperature during growth of the carbon doped layers, the resultantIII-N films grown on foreign substrates (such as Silicon substrates)have been found to have large macroscopic features on the surface. Whilethese features tend to have a fair amount of spatial separation betweenthem, devices formed directly on these features are either inoperable orperform substantially worse than other devices on the wafer.

A schematic diagram of a macroscopic feature 500 formed on the surfaceof a III-N material structure 520 grown under conditions that result ina higher density of such features is shown in FIGS. 5A and 5B. FIG. 5Ais a cross-sectional view of the feature 500, and FIG. 5B is a plan view(top view) of the feature 500. As seen in the plan view of FIG. 5B, thefeature 500 can have a hexagonal shape. The average diameter 502 of thefeatures is typically greater than 20 microns, and more specifically inthe range of about 20-500 microns, and the average height 504 of thefeature is typically greater than 100 nanometers, for example about200-500 nanometers. For comparison, in the regions of the wafer that donot include these macroscopic features, the average deviation in surfaceheight is typically much less than 20 nanometers.

Referring again to FIG. 4, it has been found that when the nucleationand/or buffer layers 404 and 406, respectively, have a carbon dopingdensity greater than 1×10¹⁸ cm⁻³, when the carbon doping is achieved byadjusting the reactor conditions, for example by lowering the surfacetemperature and reactor pressure in order to incorporate higherconcentrations of carbon into the III-N layers, the surface of the III-Nmaterial structure 420 has a density of macroscopic features 500 whichis greater than 8 features/cm². On the other hand, when the carbondoping is achieved by injecting a hydrocarbon precursor such as propaneduring growth of the layers 404 and/or 406, the density of macroscopicfeatures 500 can be made to be less than 5 features/cm², and istypically less than 2 features/cm².

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the techniques and devices describedherein. For example, the processes described herein for forming carbondoped III-N layers can be used in the fabrication of other devices thatrequire insulating or semi-insulating layers, e.g., photovoltaic cells,lasers, and LEDs. Accordingly, other implementations are within thescope of the following claims.

What is claimed is:
 1. A method of fabricating a semiconductor materialstructure, the method comprising: forming a III-N semiconductor layer ona substrate in a reactor; and while forming the III-N semiconductorlayer, injecting a hydrocarbon precursor into the reactor, therebycarbon doping the III-N semiconductor layer and causing the III-Nsemiconductor layer to be insulating or semi-insulating; wherein causingthe III-N semiconductor layer to be insulating or semi-insulatingcomprises causing the III-N semiconductor layer to have a resistivity ofat least 1×10⁵ ohm-cm.
 2. The method of claim 1, wherein injecting thehydrocarbon precursor comprises injecting a hydrocarbon precursor havinga chemical formula (C_(x)H_(y)), where x and y are integers greater thanor equal to
 1. 3. The method of claim 1, wherein forming the III-Nsemiconductor layer on the substrate comprises forming the III-Nsemiconductor layer as a III-N buffer layer over a III-N nucleationlayer over a silicon substrate.
 4. The method of claim 3, furthercomprising forming a III-N channel layer over the III-N buffer layer andforming a III-N barrier layer over the III-N channel layer, therebyforming a two-dimensional electron gas (2DEG) active channel adjacent toan interface between the channel layer and the barrier layer.
 5. Themethod of claim 4, wherein forming the III-N semiconductor layer as aIII-N buffer layer comprises forming the III-N buffer layer under aplurality of growth conditions, and wherein forming the III-N channellayer comprises forming the III-N channel layer under the same orsubstantially the same growth conditions.
 6. The method of claim 5,wherein the plurality of growth conditions comprises a surfacetemperature and a reactor pressure.
 7. The method of claim 6, whereinthe plurality of growth conditions further comprises a ratio ofgroup-III precursor flow rate to group-V precursor flow rate.
 8. Themethod of claim 4, wherein the barrier layer comprises AlGaN, thechannel layer comprises undoped or unintentionally doped (UID) GaN, andthe buffer layer comprises AlGaN or GaN or both.
 9. The method of claim1, wherein forming the III-N semiconductor layer on the substratecomprises forming the III-N semiconductor layer by metal organicchemical vapor deposition (MOCVD).
 10. The method of claim 9, whereinforming the III-N semiconductor layer comprises injecting a group-IIIprecursor into the reactor at a group-III precursor molar flow rate, andinjecting the hydrocarbon precursor into the reactor comprises injectingthe hydrocarbon precursor into the reactor at a hydrocarbon precursormolar flow rate, wherein the hydrocarbon precursor molar flow rate is atleast 0.02 times the group-III precursor molar flow rate.
 11. The methodof claim 9, wherein forming the III-N semiconductor layer comprisesinjecting a group-III precursor into the reactor at a group-IIIprecursor molar flow rate, and injecting the hydrocarbon precursor intothe reactor comprises injecting the hydrocarbon precursor into thereactor at a hydrocarbon precursor molar flow rate, wherein thehydrocarbon precursor molar flow rate is greater than the group-IIIprecursor molar flow rate.
 12. The method of claim 1, wherein thehydrocarbon precursor comprises propane.
 13. The method of claim 1,wherein carbon doping the III-N semiconductor layer results in the III-Nsemiconductor layer having a carbon concentration greater than 1×10¹⁸cm⁻³.
 14. A method of forming a semiconductor material structure, themethod comprising: forming a first III-N semiconductor layer on asubstrate in a reactor; while forming the first III-N semiconductorlayer, injecting a hydrocarbon precursor into the reactor, therebycarbon doping the first III-N semiconductor layer and causing the firstIII-N semiconductor layer to be insulating or semi-insulating; andforming a second III-N material layer on the first III-N semiconductorlayer; wherein the second III-N material layer has a substantially lowercarbon concentration than the first III-N material layer.
 15. The methodof claim 14, wherein the substrate is a foreign substrate.
 16. Themethod of claim 15, wherein a surface of the second III-N material layerthat is opposite the substrate has a density of macroscopic featureswhich is less than 5 features/cm², wherein each of the macroscopicfeatures has an average height of greater than 100 nanometers.
 17. Themethod of claim 14, wherein the hydrocarbon precursor is injected intothe reactor while forming the first III-N material layer but is notinjected into the reactor while forming the second III-N material layer.18. The method of claim 17, wherein the hydrocarbon precursor comprisespropane.
 19. The method of claim 17, wherein the second III-N materiallayer is thinner than the first III-N material layer.
 20. A materialstructure, comprising: a first III-N semiconductor layer on a foreignsubstrate; and a second III-N semiconductor layer on a side of the firstIII-N material structure opposite the foreign substrate, the secondIII-N semiconductor layer being thinner than the first III-Nsemiconductor layer; wherein the first III-N semiconductor layer has acarbon concentration greater than 1×10¹⁸ cm⁻³ throughout the layer; acarbon concentration throughout the second III-N semiconductor layer isless than the carbon concentration throughout the first III-Nsemiconductor layer; and a surface of the second III-N material layerthat is opposite the substrate has a density of macroscopic featureswhich is less than 5 features/cm², wherein each of the macroscopicfeatures has an average height of greater than 100 nanometers.
 21. Thematerial structure of claim 20, the second III-N semiconductor layercomprising a III-N barrier layer and a III-N channel layer, wherein atwo-dimensional electron gas (2DEG) active channel is adjacent to aninterface between the III-N channel layer and the III-N barrier layer.22. The material structure of claim 20, wherein a combined thickness ofthe first III-N semiconductor layer and the second III-N semiconductorlayer is less than 6 microns.
 23. A material structure, comprising: afirst III-N semiconductor layer on a foreign substrate; and a secondIII-N semiconductor layer on a side of the first III-N materialstructure opposite the foreign substrate, the second III-N semiconductorlayer being thinner than the first III-N semiconductor layer; whereinthe first III-N semiconductor layer is an insulating or semi-insulatinglayer having a carbon concentration greater than 1×10¹⁸ cm⁻³; a carbonconcentration of the second III-N semiconductor layer is less than thecarbon concentration of the first III-N semiconductor layer; and adislocation density at a surface of the second III-N semiconductor layeropposite the foreign substrate is less than 2×10⁹ cm⁻².
 24. The materialstructure of claim 23, wherein a combined thickness of the first III-Nsemiconductor layer and the second III-N semiconductor layer is lessthan 6 microns.